This invention relates to a method of manufacturing steel members such as transmission gears of automobiles which require superior strength property.
Recently, with the rise of powerful, light and simplified engines, it has been urged to manufacture steel members used for transmission gears of automobiles which can meet the demand of higher bending fatigue strength and higher pitting strength as well as superior strength.
The following methods have been known as the conventional manufacturing processes of steel members requiring superior strength. The first method comprises the steps of carburizing and quenching or carbo-nitriding and quenching a steel material, and shot-peening said steel material. The second method, as disclosed in Japanese Patent Application Laying Open Gazette No. 60-218423, comprises the steps of tempering a steel material after heat treatment, shot-peening said steel material, and re-tempering said steel member at a temperature lower than that in the tempering step. The third method, as disclosed in Japanese Patent Application Laying Open Gazette No. 62-207822, comprises the steps of tempering a steel member after carburizing and quenching, shot-peening said steel member, and aging said steel member.
The first method mentioned above is to improve fatigue strength of a steel material by shot-peening, that is injecting shots on the surface of a steel material to strengthen the surface layer thereof with a surface-hardening treatment, and to create compressive residual stress in the surface of the steel material. However, in this method, sufficient pitting strength required for steel members used in transmission gears cannot be achieved.
The second method is to improve fatigue strength by retempering a steel material to precipitate .epsilon.-carbide therein. However, since a steel material is tempered before shot-peening, this method results in a decreased amount of retained austenite in the steel material. Consequently, shot-peening after tempering does not lead to obtaining enough compressive residual stress in the steel material, thus failing to achieve improvement of fatigue strength. In this method, sufficient pitting strength required for steel members used in transmission gears cannot be obtained, as in the first method.
In the third method, since a steel material is tempered before shot-peening as in the second method, this third method results in a decreased amount of retained austenite in the steel material. Therefore, sufficient improvement in fatigue strength of the steel member cannot be achieved. As for pitting strength, the same is applied to the third method as in the second method.
The object of the present invention is to provide a method of manufacturing a steel member which can improve pitting strength as well as fatigue strength thereof.